Molecular frequency standard

ABSTRACT

A molecular beam source for a molecular beam tube frequency standard using a barium oxide molecule of the form Ba138016 having spinless atoms and using electrostatic state selection, which source includes a thin-walled refractory electrically conductive oven tube containing Ba138016. The oven tube, which is chemically inert to barium oxide, is surrounded by a thermionic electron emitter which is made negative with respect to the oven tube. Electrons emitted from the electron emitter bombard the oven tube and heat the barium oxide to a temperature at which Ba138016 molecules are evaporated.

v D United Si Hellwig June 6, 1972 Sterzl...............

agovitz, Edward J. Kelly, Herbert Ber y m e. M h GI. 3m m mE mm wa T e .m mmm mm h xH amtw mm mmwd P mm m y m m m m MM m am a n mmm .W ue Sm m m m M 8m m um n a m w. v m h A T. n .U U

[22] Filed: Mar. 17, 1970 ABSTRACT U- Application Data A molecular beam source for a molecular beam tube frequen- 5 5 m mm 4 m u P ,5 303 8 6 n 9 M W l 0 5 i 5 2 6 2 2. l l 3 7 m 0 m N m.m a S "mm 1. A O8 "0, n 0 .l m mm d .w5, D3 UIF l. 1.1] 2 2 8 6 555 .l. [[l.

ndhn mmm mfnm cmfldon m kr w w hto fvde la Tr c u 01 8 fi bnm t km awmo 'w afi B mm kg" 0 d onwm tmm mummmm m m w mmam O Scm m e mt mm w hm umh mml mamt.wieE nln fieedow k CUu Oil t m.0emhn mS .W meW i s e i i mmwnh ah kmo d nc t 60 8 m C wu t S Cd 8 wbb-w m cBmmmbm I emitter bombard the oven tube and heat the barium oxide to a References Cited temperature at which 821 0 molecules are evaporated.

UNITED STATES PATENTS $9 3 9 2,. a

5 Claims, 3 Drawing figures yrp t FE v minnow" s 1272 3,668,293

INPUT OUTPUT 34 FIR T4 OVEN l2 RESONATOR SECOND HOT WIRE HEATER COIL l4 ENERGY STATE CAVITY ENERGY STATE DETECTOR AND SELECTOR SELECTOR MULTIPLIER ELECTRODES ELECTRODES TNVENTOR HELMUT W. HELL'NFS ATTORNEYS MOLECULAR FREQUENCY STANDARD This application is a division of my prior copending parent application, Ser. No. 721,776, filed Apr. 16, 1968, which is now US. Pat. No. 3,578,968, issued May 18, I971.

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

For example, the relatively heavy weight of the Ba molecules is a factor in achieving a relatively low molecular beam velocity of the order of 4X10 centimeters per second, thus minimizing the effect of beam velocity on the accuracy of the frequency standard--.

In the case of the Ba"0 molecule, the transition frequency of 18.7 GI-Iz is within the promising frequency range of about to 30 GI-Iz because this frequency range yields a relatively good line-Q while still permitting a cavity type resonator of reasonable size and use of well known microwave generators of high efficiency and performance rating. By way of comparison a CO beam has a much higher transition frequency of 115 GHz and would impose greater restrictions on electromagnetic generators and resonators.

Also, the high evaporation temperature of barium oxide, which is a solid at ordinary room temperatures, results in a vapor pressure at ambient temperatures which is extremely low. Consequently, vacuum problems with this particular barium oxide molecular beam tube are virtually nonexistent; in contrast, many molecules exist in the gaseous phase at ordinary room temperatures.

In order to obtain adequate beam strength, a-relatively high vapor pressure of about 10' Torr or greater is needed; this, in turn, requires a relatively high temperature (of the order of l500C.) in the case of barium oxide. For a practical molecular'beam frequency standard, the molecular beam generating oven must be of small size and low energy consumption. The oven itself is an iridium tube containing the barium oxide in solid form; the iridium tube is closed off except for a small opening in one end to allow exit of the molecular beam. The iridium tube is surrounded by a conventional thermionic emissive low voltage heater element which is made negative with respect to the iridium tube by'means of an appropriate low voltage power supply expending about 50 watts. Electrons emitted from the heater element bombard the positive oven tube, thereby heating the thin oven tube and the barium oxide contained therein to a temperature sufficient to evaporate ofi molecules of barium oxide. Radiation heat losses are minimized by a series of surrounding heat shields and conduction heat losses are limited by use of a thin-walled refractory tube for mounting the iridium oven tube. Iridium is chosen for the oven tube because it is a highly refractory metal which is chemically inert to barium oxide. The heater tube must be a metal since it must be electrically conductive to attract electrons emitted from the thermonic heater element; this rules out such refractory materials as ceramics. Since the temperatures of theorder of 1,500" C or greater must be obtained to evaporate the barium oxide, the oven tube must be made of metal having a melting point in excess of l,500 C. Use of a refractory metal such as tungsten is precluded since such a container would disintegrate from the oxidation reaction occurring between tungsten and barium oxide. An iridium oven tube, on the other hand, is not contaminated or physically destroyed by contact with the evaporating Ba' 0 SUMMARY OF THE INVENTION A beam source for a molecular beam tube frequency standard comprises a thin-walled refractory oven tube of iridium containing barium oxide to be evaporated. The iridium oven tube, which does not react chemically with the barium oxide, is surrounded by an electron source such as a tungsten cathode which is electrically heated. A direct current voltage is applied between the source and the oven tube such that the oven tube forms the anode; consequently, electrons from the cathode bombard the oven tube and the material in the oven tube is heated by electron bombardment, whereby a beam of molecular barium oxide is produced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view showing schematically a molecular beam tube according to the invention;

FIG. 2 is a view showing a beam generating source according to the invention;

FIG. 3 is a view showing in detail the heater wire assembly of the beam generating source of FIG. 4; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates schematically a molecular beam tube 5 according to the invention. The molecular beam is generated by evaporation of Ea o in an oven 10 which comprises an iridium heater tube 12 containing the barium oxide in solid form. The heater tube 12 is surrounded by heater wires 14 which is negative with the respect to the heater tube so that the electrons emitted from the heater wire bombard the heater tube 12 to heat the latter to the required evaporation temperature of the barium oxide. The molecular beam then enters a first state selector 15 which can be of the quadrupole type designed for optimum focusing of the Ba 0 in the upper sublevel J=1,m=0 with a most probable speed of about 4.3 l0" centimeters per second (corresponding to an oven temperature of 1,500 C.) when operated at 5 kilovolts. This quadrupole focuser or state selector 15 comprises four angularly electrodes or wires (only two of which are visible in FIG. 1), with alternating ones being interconnected. A high voltage is applied in push-pull to the alternate electrodes or wires. The molecule of the selected state are concentrated along the axis of the tube by the focusing effect produced in the state selector 15. The other states of lower energy level are deflected away from the longitudinal axis of the beam tube 5 and do not enter the cavity resonator. Input microwave energy of the proper transition frequency (13.702 GHz) is supplied to a Rabi cavity resonator 20 from any conventional microwave generator such as a klystron, not shown. This resonator 20 is designed to operate in the TM made so as to obtain substantial interaction between the electric field within the microwave cavity resonator and the molecular particles passing through the cavity resonator. The latter is designed, in a manner already described in some detail, to minimize mechanical and electrical irregularities which tend to produce phase and Doppler shifts within the cavity resonator. The Ba O molecules upon leaving the cavity resonator, pass through a second electrostatic state selector or focuser 25 which may be identical to the first state selector 15. The molecules of the desired J=l, M=0 state are converged by the state selector 25 onto the axis of the molecular beam tube while molecules of other energy levels are deflected away from the axis of the molecular beam tube 5. Because the molecular beam has a Maxwell velocity distribution, the second state selector 25 will have a focal region along the axis of the beam tube rather than a single focus. A substantial portion of the molecules of the selected state impinge upon a detector 30, for example, can comprise an axial hot wire, heated to a relatively high temperature and impinged upon by a substantial portion of the barium oxide molecules; the ions resulting from said impingement are collected output current is provided which is a minimum whenever the cavity resonator excitation coincides with the resonant transition frequency.

FIG. 2 illustrates a beam generating oven 10 in accordance with the invention. The oven itself is a small, thin-walled iridium tube l2 close at one end 39 and a centrally apertured plug 41 serving as a chamber for retaining the Ea o material to be evaporated. The reasons for the choice of iridium for the oven tube already have been set forth. The plug 41 is centrally apertured to allow exit of the molecular beam. The iridium oven tube 12 is fixedly attached to an elongated hollow support tube 44 made of a refractory metal such as tantalum. The support tube 44 is thin-walled so as to minimize conduction heat losses from the iridium tube 12. The support tube is attached to the oven end plate 42, which also serves as an end inclosure for the molecular beam tube 5; this end plate 42 can be made, for example, of stainless steel. The support tube 44 also provides support for the heater wire assembly 45 which surrounds the iridium oven tube 12. The oven tube 12 is recessed in the heater assembly 45 so as to provide for maximum heating at the oven tube aperture. The heater wire assembly 45 includes a hollow cylindrical member 46 of a refractory metal such as tantalum with mica inserts 47 and 48 at each end. The support tube 44 passes through a central aperture in mica insert 47 and is supported therein. The heater wire 14 is in the form of a cage-like structure in which the heater includes a plurality of longitudinally arranged portions which pass through one pair of aligned apertures in the mica spacers 47 and 48 and loop over a mica space to the next pair of aligned apertures, as shown in FIG. 3. One end 50 of heater wire 14 is connected by way of one heater terminal 53 to the negative terminal of an external direct current power supply 55 through one end of transformer 56. The other end 51 of the heater wire 14 is connected by way of a terminal post 54 to the opposite end of the aforesaid transformer. The positive terminal of the power supply 55 may be connected directly to the envelope 9 of beam tube which is in electrical contact with the support tube 44 and oven tube 12. The terminal post 53 and 54 are brought through the end plate 42 by respective electrically insulating bushings 58 and 59.

In order to reduce heat radiation loses from the oven, a tripartite heat shield assembly surrounds the heater wire assembly 45. The composite heat shield includes an inner shield 62 supported from the middle heat shield 63 by electrically insulating supports. The middle heat shield 63 is attached to the outer heat shield 64, as by brazing, and the entire heat shield assembly is supported by connecting an elongated portion of the outer heat shield 64 to the end plate 42 and by means of a thin annular disc 57 which is attached to the inner periphery of the tube envelope 9. The heat shields preferably are made from polished stainless steel and reflect back to the oven a substantial portion of the heat radiated from the oven.

The oven tube 12, together with the support tube 44, are at a positive potential relative to the alternating current heater wire 14. The heater wire 14 can be of the type commonly used in low power indirectly heated vacuum tube rectifiers; the input power required of such a heater wire is only a few watts. A voltage of about 250 volts exists between the heater element 14 and the oven tube 12. Electrons emitted thermionically from the hater wire 14 with a current of about 200 milliamperes are attracted to the positive oven tube 12 and the electron bombardment of this iridium tube is such as to bring the temperature thereof to about 1,500" C. This temperature is well below the melting point (2,410 C.) of the iridium tube 12. Molecules of l3a""0" evaporated from the oven pass through the exit aperture in the oven chamber at a velocity of about 4.3 l0 centimeters per second at the temperature of 1,500 C. A satisfactory signal-to-noise ratio is obtainable with 10" molecules per second arriving at the detector. To provide this number of molecules, a typical beam tube would require a molecular flux from the oven source of about 10 molecules per second. The volume of the iridium oven is about 0.25 cu. centimeters so that one charge of Ba tl' contains about 5Xl0 molecules enough for 17 years of continuous operation at the aforesaid beam intensity. The end disc 69 and the end portions of the three heat shields 62, 63 and 64 are apertured centrally to permit passage of the molecular beam and also can contain non-aligned apertures to facilitate evacuation of the tube prior to operation. Once a vacuum has been obtained and the tube closed off there is no need for a vacuum pump since the vapor pressure at room temperature for B29 0 is negligible of. the order of 10 Torr and the only gas generating processes in the beam tube 5 would result from outgassing. This can be taken care of readily, however, by insertion of a conventional getter in the vicinity of the oven 10.

lclaim l. A beam generating source comprising a thin-walled refractory oven tube containing a solid material to be evaporated, said oven tube being impervious to chemical reaction with said material, said oven tube being surrounded by a thermionically emissive electron source heated to emit electrons, and means for maintaining said oven tube positive with respect to said electron source, said emitted electrons bombarding said oven tube and heating said oven tube and said material to a temperature sufiiciently high to evaporate a portion of said material from said oven tube.

2. A beam generating source according to claim 1 further including a heat shielding structure surrounding said electron source for minimizing heat transfer from said oven tube.

3. A beam generating source according to claim 1 wherein said electron source and said means for maintaining are of low energy consumption.

4. A beam generating source according to claim 1 wherein said oven tube is iridium.

5. A beam generating source according to claim 1 wherein said solid material is barium oxide in the form Ba O. 

2. A beam generating source according to claim 1 further including a heat shielding structure surrounding said electron source for minimizing heat transfer from said oven tube.
 3. A beam generating source according to claim 1 wherein said electron source and said means for maintaining are of low energy consumption.
 4. A beam generating source according to claim 1 wherein said oven tube is iridium.
 5. A beam generating source according to claim 1 wherein said solid material is barium oxide in the form Ba138O16. 